The growth of mammalian cells in synthetic media depends on the presence of a "serum growth factor". We found this factor in all mammalian sera (bovine, horse, human) and in all (Cohn) fractions of sera, however, varying in strength and quantity. It is not one but two substances which, together, promote "growth control in cell cultures". Our first goal is to isolate these two substances and to prove in microbiological tests that, if combined in the right proportion, their activity equals that of the most potent sera. In the sera, and principally in those of older animals and humans, there also exist inhibitors which we isolated by enzymatic splitting of serum proteins. Their structure is completely different from that of the "growth factor" and their inhibitory activity can be tested on cell cultures, together with the "growth factor". The methods applied to isolate both the "growth factor" and the inhibitors from the sera of healthy animals and humans include salt precipitation, chromatography and electrophoresis, as well as filtration with Amicon filters and Hollow fibers. After having established the chemical and biological properties of the growth-promoting and inhibiting serum factors, our most important aim will be to find and study the same or similar factors present in cancer tissue.